The present invention relates to new ligands affording enhanced catalytic activity of metal complexes and the relationship between the characteristics of these ligands and the catalytic properties of the corresponding complexes. In particular, the present invention relates to rhodium metallacrown ethers and methods of hydroformylation catalysis therewith.
One of the most important industrial processes using homogeneous catalysts is the hydroformylation of alkenes to produce aldehydes:
RCH=CH2+CO+H2xe2x86x92RCH2CH2CHO linear (n-product)+RCH(CHO)CH3 branched (iso-product).
In this reaction, the elements of formaldehyde are added across a double bond to produce a mixture of linear (n-) an branched (iso-) products. Linear aldehyde products are bulk starting materials for plasticizers and detergents and have consequently secured the major market demand for industrial production. Chiral iso-aldehyde products, which are formed in asymmetric hydroformylation reactions, are in demand as enantiomerically pure precursors for pharmaceuticals and agrochemicals (1-5).
The most widely used hydroformylation catalysts are derivatives of HRh(CO)(Ph3P)3. Reactions using these rhodium-based catalysts are run under mild conditions (60-130xc2x0 C., 10-60 bar) with high n/iso selectivity and low byproduct formation (2,6). The best results are obtained with linear terminal alkenes of short to medium length (C3-C12). For long-chain or sterically hindered alkenes, hydroformylation catalysts with bulky, chelating bis(phosphite) ligands have high activities (1,2,7). Catalysts containing bulky and chelating bis(phosphite) ligands have also been effective in asymmetric hydroformylation. As an example, Union Carbide has reported that a catalyst with a bis(phosphite) ligand exhibits an enantioselectivity of 90% and an n/iso ratio of 1:50 for asymmetric hydroformylation of substituted styrene derivatives (2,8,9). Similarly promising results have been seen for a phosphine-phosphite hybrid ligand (95% ee, n/iso ratio of 14:86) (2,10).
Hydroformylation reactions utilizing rhodium catalysts have been studied extensively, and general mechanistic ideas are well established. However, each catalytic system and each set of structurally related substrates seems to yield a unique combination of products (e.g. ratios of isomers). Reaction conditions exert a major influence on the product outcome, and thus the comparison of results and elucidation of trends between studies are difficult. The desire to better understand hydroformylation coupled with the commercial utility of the products explains the continuing interest in this reaction (11-15).
Ligand design for hydroformylation catalysts remains empirical in nature because no adequate relationships between the structural/electronic properties of the phosphorus-donor ligands and the catalytic activities/selectivities of their rhodium complexes have been developed (6,15). It has been demonstrated that changes in the steric or electronic properties of the phosphorus-donor ligands can have significant effects on the catalytic activity and product selectivity of their rhodium complexes (2,6,15-18). This suggests that the development of phosphorus-donor ligands with functional groups that can interact with intermediates in the catalytic cycle is a viable approach to the development of new hydroformylation catalysts.
McLain has reported that rhodium complexes of a ligand incorporating a diphenylphosphino group tethered to an azacrown ether exhibit increased rates of hydroformylation of alkenes in the presence of Na+ or Li+ (20). Studies of model complexes for the acyl intermediates in the catalytic cycle demonstrated that Na+ can coordinate to both the acyl oxygens and the azacrown ether (20-22). This type of interaction is known to accelerate alkyl migration to coordinated carbon monoxide (23,24).
McLain""s catalysts are limited in utility owing to the ligands. Complexes with unsymmetrical, chelating bis(phosphorus-donor) ligands, can exhibit much higher catalytic selectivities than do those with monodentate ligands (2,19,25). Electron-poor phosphorus-donor ligands may have significant advantages over electron-rich phosphines in hydroformylation catalysts (2). Further, the McLain""s ligands are monodentate, thereby requiring an excess of azacrown ether groups in the catalysts.
Metallacrown ethers are a class of receptors formed by the chelation of xcex1,xcfx89-bis(phosphorus-donor)polyether ligands to transition metals (26). A wide range of these metallacrown ether complexes with a variety of metal centers, phosphorus donor groups (both electron-rich and electron-poor), and polyether binding sites have been prepared. All metallacrown ethers contain a single hard metal cation-binding site close to the metal center.
Carbonyl ligands in metallacrown ethers are activated toward nucleophilic attack by alkyllithium reagents (27-31). It also has been shown that Hg2+ catalyzes the cis-trans isomerization of cis-Mo(CO)4 metallacrown ethers (32).
The hydroformylation of alkenes to produce aldehydes, catalyzed by rhodium complexes of phosphorus-donor ligands, is an important industrial route to precursors for plasticizers and detergents. There exists a need for more active catalysts and for catalysts that work with a wider range of substrates.
A metallacrown ether ligand has the formula 
where Y is "Brketopenst"Zxe2x80x94(xe2x80x94CR6R7)n"Brketclosest"mxe2x80x94Zxe2x80x94R5"Brketopenst"Zxe2x80x94(xe2x80x94CR6R7)n"Brketclosest"m, xe2x80x94R5"Brketopenst"Zxe2x80x94(xe2x80x94CR6R7)n"Brketclosest"mZxe2x80x94, xe2x80x94Zxe2x80x94R5"Brketopenst"Zxe2x80x94(xe2x80x94CR6R7)n"Brketclosest"m, xe2x80x94Zxe2x80x94R5"Brketopenst"Zxe2x80x94(xe2x80x94CR6R7)n"Brketclosest"mxe2x80x94Zxe2x80x94, n is an integer between 1 and 6 inclusive, m is an integer between 1 and 8 inclusive, Z is oxygen or NR9, R1-R4 are each independently C1-C6 alkyl, C1-C6 alkoxyl, C2-C6 alkenyl, aromatic cyclics and substituent-containing forms thereof where the substituent is C0-C6 alkyl, fluoro, chloro, bromo, hydroxyl, carboxyl, sulfonyl and other nitrogen, oxygen or sulfur containing moieties; or two of the groups R1, R2, R3 and R4 are fused to form biaryl, biphenoxy, binaphtoxy, phenanthrenoxy and substituent-containing forms thereof where the substituent is C0-C6 alkyl, fluoro, chloro, bromo, hydroxyl, carboxyl, sulfonyl and other nitrogen, oxygen or sulfur containing moieties, R6, R7 and R9 are each independently hydrogen, C1-C6 alkyl, C1-C6 alkoxy heteroatom substituted C1-C6 alkyl where the heteroatom is O, N, S, F, Cl and Br, two of R6, R7 and R9 are fused with the common carbon center to form a 3-6 carbocyclic ring structure, and R5 is xe2x80x94ZR8xe2x80x94, biphenoxy-R8, binaphtoxy-R8, phenanthrenedioxy-R8, anthracenedioxy-R8, "Brketopenst"Zxe2x80x94(xe2x80x94CR6R7)n"Brketclosest"m biphenoxy R8, "Brketopenst"Zxe2x80x94(xe2x80x94CR6R7)n"Brketclosest"m binaphtoxy R8, "Brketopenst"Zxe2x80x94(xe2x80x94CR6R7)n"Brketopenst"m phenanthrenedioxy-R8 and "Brketopenst"Zxe2x80x94(xe2x80x94CR6R7)n"Brketclosest"m anthracenedioxy R8, biphendiamino-R8, binaphtdiamino-R8, phenanthrenediamino-R8, anthracenediamino-R8, "Brketopenst"Zxe2x80x94(xe2x80x94CR6R7)n"Brketclosest"m biphendiamino R8, "Brketopenst"Zxe2x80x94(xe2x80x94CR6R7)n"Brketclosest"m binaphtdiamino R8, "Brketopenst"Zxe2x80x94(xe2x80x94CR6R7)n"Brketclosest"m phenanthrenediamino-R8 and "Brketopenst"Zxe2x80x94(xe2x80x94CR6R7)n"Brketclosest"m anthracenediamino R8 and substituted forms thereof where the substituent is C0-C6 alkyl, fluoro, chloro, bromo, hydroxyl, carboxyl, sulfonyl and other nitrogen, oxygen or sulfur containing moieties; and two of the groups R1, R2, R3 and R4 are fused to form biphenoxy, binaphtoxy, phenanthrenoxy and substituted forms thereof where the substituent is C0-C6 alkyl, fluoro, chloro, bromo, hydroxyl, carboxyl, sulfonyl and other nitrogen, oxygen or sulfur containing moieties. R8 is C1-C6 alkyl, C1-C6 alkoxyl, C2-C6 alkenyl, aromatic cyclics and substituent-containing forms thereof where the substituent is C0-C6 alkyl, fluoro, chloro, bromo, hydroxyl, carboxyl, sulfonyl and other nitrogen, oxygen or sulfur containing moieties. A ligand according to Formula (I) chelates a rhodium atom to form a rhodium metallacrown ether catalyst.
A process of catalyzing unsaturated substrate hydroformylation includes the step of exposing an unsaturated substrate to carbon monoxide and hydrogen in the presence of an effective amount of a metallacrown ether catalyst having a ligand of Formula (I). The use of a ligand of Formula (I) is also contemplated to chelate a metal for the preparation of a metallacrown ether for a catalytic application.